WO 2010/046811 A1 discloses an apparatus and a method for improving the uniformity of at least one optical property of a tiled organic light emitting diode (OLED) lighting source comprising at least two OLED tiles. Electrical power is applied to the OLED tiles with a power providing means, wherein the power providing means comprises a control means adapted for controlling the electrical power to each of the OLED tiles. At least one optical property of each of the OLED tiles is measured as a function of their respective electrical power to determine at least one electro-optical property of each OLED tile. The control means is modified by using the electro-optical properties for compensating the effect of a variation of the electro-optical properties on the uniformity of the optical properties of the OLED tiles.